______ In-class notes for 09/16/2020 (CS 273 (OS), Fall 2020)
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In-class notes for 09/16/2020

CS 273 (OS), Fall 2020

  • Shell project

  • Today - IPC before proceeding to in-person Virtual Box lab (Friday)

    • Will send google doc (via Piazza) to determine who else be in-person, who remote

    • Seek to include everyone with prerequisites only background (who wants to come)

    • Safety for in-person help: TA(s) and I will have face shield + mask, with time limits for close proximity

    • Look on Piazza for prep steps to carry out before Friday class (e.g., software download)

IPC

Example problem: thread-safe array

  • A shared data structure - any thread should have access to read or write elements of that array.

  • Risk of race condition for each element.

  • Basic operations needed:

    • set_array(array, index, value)    - compare to assignment   array[index] = value

    • get_array(array, index)           - compare to value of   array[index]

  • Exercise: Assuming an implementation of semaphores in your OS, give a pseudocode implementation of:

    • set_array()

    • get_array()

    Note: Be sure to specify the initial value of any semaphore you use.

    If time: check that your solution satisfies the four goals for correct IPC.

Semaphores

  • Review:

    • Data structures - each semaphore has a non-negative integer variable and a queue of blocked processes

    • Primitive operations are down(sem) and up(sem), implemented in OS as system calls

    • Example: re-implementing phtreads.c to use semaphores instead of pthread locks.

Monitors

  • Summary of features:

    • A monitor is a programming-language entity, similar to an object

    • Mutual exclusion is guaranteed for a monitor's "methods" - no two threads can execute methods for the same monitor at the same time

    • Special data structures called condition variables, which are queues of blocked processes

    • Primitive operations wait(condvar) and signal(condvar)

      • We will assume that both of these calls relinquish mutual exclusion
        but caller of wait() gets mutual exclusion back when it becomes unblocked (i.e., by another process calling notify()

  • Exercise: Assuming monitors in your programming language, give a pseudocode implementation of:

    • set_array()

    • get_array()

    If time: check that your solution satisfies the four goals for correct IPC.

Message Passing

  • Summary of features:

    • Message passing involves transmitting data between processes/threads that could be on other (networked) computers

    • Data structures are messages (data to be transmitted) and a queue of pending messages for each destination (process/thread that can receive a message).

    • Primitive operations send(dest, msg) and receive(src, &msg), for transmitting a message msg from a source process src to a destination process dest.

      • We will assume that send() is non-blocking and receive() is blocking (calling process blocks if there is no message to receive).

    • Unlike semaphores and monitors, note that message passing does not require that all processes run on the same computer.

  • Exercise: Assuming message passing between processes/threads, give a pseudocode implementation of:

    • set_array()

    • get_array()

    Hint: Use a separate process/thread for managing the shared data structure.

    If time: check that your solution satisfies the four goals for correct IPC.

Implementations of producer-consumer problem

To study for quiz

  • Process diagrams, including redirection of output, pipelines, etc.

  • Process states -- be ready to discuss how OS services (e.g., I/O operations, scheduling, system calls such as wait(), interrupt handling) relate to the process-state diagram

  • Algorithms and data structures for processes:

    • Process table -- Array of process table entries; each PTE contains information relevant to a particular process. Have a general familiarity with type of information (see examples).

    • Interrupt vector

    • Algorithm for handling interrupts -- study these steps, and ask any questions on Piazza or on questions of the day for next class meeting.

  • Programming with system calls: be prepared to write segments of C code that may use any of the following system calls: fork(), execve(), wait()




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rab@stolaf.edu, September 18, 2020